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WARNING, GRAPHIC CONTENT | VIDEO: Nigerian President Muhammadu Buhari Tweeted that his government will leave "no stone unturned" investigating charges of widespread torture, abuse and killing of more than 8,000 prisoners during the military’s offensive against the terror group Boko Haram, according to a recent repory by Amnesty International. Henry Ridgwell reports from London.

Nigerian President Muhammadu Buhari on Thursday said his government will leave "no stone unturned" investigating allegations of widespread torture, abuse and the killing of more than 8,000 prisoners during a military offensive against Boko Haram militants.

Buhari tweeted the announcement in response to Amnesty International's Wednesday report that drew from dozens of brutal videos and images showing abuses and killings apparently carried out by the Nigerian armed forces. The London-based rights group has named five senior officers who it says should face investigation.

In footage supplementing the 133-page report, prisoners are beaten to death while others are shot and thrown into open graves. Amnesty officials say the research draws from interviews with more than 400 victims, eyewitnesses and senior members of security forces.

Most of the prisoner deaths occurred during detention, says Amnesty’s Daniel Eyre.

“We have found that more than 7,000 people have died in military custody in northeast Nigeria in some of the most appalling conditions imaginable," he said. "They died of starvation, of thirst, as a result of lack of access to medical care. Many were tortured.”

Amnesty’s evidence also points to the widespread extrajudicial killing of prisoners. Some were hung on poles and suspended over fires; others were thrown into pits, had their throats cut, or were shot.

“More than 1,200 men and boys were unlawfully killed by the Nigerian military," he said. "That includes cases like what happened after the Giwa Barracks attack, where more than 640 recaptured detainees were executed in one day. And our report shows that the people at the top of the Nigerian military knew about these crimes and did not take action to stop them.”

The charges overshadowed Buhari's first trip abroad as president. He vow to "to promote the rule of law and deal with all cases of human rights abuses," during a stop in neighboring Niger to discuss Boko Haram and pledge a new strategy to fight the terrorists.

"Five years of the evil presence of this sect is enough," he told Niger officials on Wednesday, adding that although Nigerian soldiers have suffered many defeats, it is not the moment to criticize.

Furthermore, he said, "we are going to mobilize our best forces to attack this sect."

The Amnesty report names five senior officers who it says should face investigation.

On Thursday, Major General Chris Olukolade, Nigeria’s Director of Defense Information, accused Amnesty of trying to blackmail the military and said the five senior officers named in the report have "no reason" to indulge in the allegation made against them.

But evidence supporting the charges, says Eyre, is compelling.

“In fact, at one point they commissioned a report, which showed the appalling conditions in detention, the disease that was causing these deaths," he said. "And yet they did not take action to put an end to these deaths. And that is why we are saying they need to be investigated for their responsibility.”

The United States, which had strained ties with President Buhari’s predecessor Goodluck Jonathan, has signaled its readiness to expand military cooperation in the fight against Boko Haram. But analysts say Nigeria’s Western allies will want the allegations of war crimes to be fully investigated.